My betta is in a 5 gallon, and I'm looking for tankmates for him after the tank is fully cycled. I want to get either neons or otto cats but I was wondering which would be easier to take care of? And, are there any other tankmates that are easily taken care of which are compatible with bettas? Thanks!

A 5 gallon is a great tank size, but sadly it is too small for actual fish tankmates. Both Ottos and Neons are schooling/shoaling fish who need to be in larger groups of 5-6+, which is just too much for a 5 gallon. There just isn't enough swimming space.
Ottos can be a little finicky anyway. It can be hard to get them to eat and they're really sensitive to water quality. I believe this is because most of them are wild-caught...

With a single betta in a 5 gallon, you're pretty limited with tank mates. You could do a couple of snails or a few shrimps(Ghost shrimp are the ones most commonly sold, though you can look for Red Cherry shrimp or a few other species online if you want something a tad more colorful.....careful though! Bettas often make shrimps snacks). Or if you'd be willing to deal with the tricky feeding, you could go with a pair of ADFs(African Dwarf Frogs).
Another option is dividing the tank in half for a second betta.....but thats about it I'm afraid.

If you are going to divide like Dragon fish said be very careful. If you are going the divide it with glass or another solid surface that's fine, but if you are using a divider that water can get through be careful.
If you make a divider, and use binders and mesh, make sure the binder rings are completely covered. Or better yet don't use this method.
My betta fish recently got stuck in between the rings and is now recovering from ripped ventral fins and wounds on his body. I don't want this to happen to your fish.
Sorry for sounding rather depressing and morbid.
Good luck on your tank mate search, Im just about to begin researching that myself!